Remove boot class path

I am suppressing the code which adds the bootclasspath attribute to the generated agent's manifest.mf file. First, @Databean's experience (see his comments in #60) shows that the agent doesn't really need it. Second, according to this [sun official document](http://docs.oracle.com/javase/7/docs/api/java/lang/instrument/package-summary.html), that attribute is completely optional. Third, this is causing some unnecessary warnings to be generated in windows. 

@pdaian, @Databean, @seriousamlqz, please review